1. Field of the Invention
The present invention relates to a method of manufacturing a fuel cell bipolar plate, and more particularly, to a method of manufacturing that enables a fuel cell bipolar plate having superior dimensional accuracy to be manufactured efficiently.
2. Background of the Invention
A type of fuel cell that generates electric power by the electrochemical reaction of a fuel gas and an oxidizing gas, in particular a solid polymer type fuel cell, shows promise as a clean, renewable power source in a variety of applications. Such a fuel cell is comprised of an ion-conducting electrolyte membrane sandwiched between an anode and a cathode, the anode and cathode each composed of a gas diffusion electrode coated with a catalyst, the outsides of the electrodes being further provided with a bipolar plate. The bipolar plate at the anode supplies hydrogen as the fuel gas to the anode and the bipolar plate at the cathode supplies oxygen as the oxidizing gas to the cathode.
FIGS. 8A and 8B show an example of such a bipolar plate. As shown in the diagrams, a pattern 1a consisting of narrow grooves created by variations in thickness is formed on a planar surface of a fuel cell bipolar plate 1. In order to increase the overall surface area of contact between the gas diffusion electrode and the gas, the grooves meander with a slight pitch over the entire surface of the fuel cell bipolar plate. The groove pattern 1a may be formed on both sides of the fuel cell bipolar plate, as shown in FIG. 1B, or on one side only.
In addition to the configuration described above, there are also fuel cell bipolar plates having other types of structures, in which the projections are arrayed on one or both sides of the bipolar plate and the gaps between the projections used as passages for gas, or in which a combination of projections and the configuration described above is used.
The fuel cell bipolar plate described above requires the following characteristics:
(1) Gas impermeability. The fuel cell bipolar plate must be impermeable to the hydrogen and oxygen gases supplied to the electrodes. Typically, a fuel cell is formed into a cell stack consisting of many individual cell units stacked together, with each cell unit consisting of a central solid polymer-type electrolyte membrane, gas diffusion electrodes on both sides of the electrolyte membrane, and bipolar plates outside both electrodes. Therefore, gas is flowing to at least one side of the fuel cell bipolar plate, and if the bipolar plate were gas-permeable it would degrade the power generating efficiency of the cell or render power generation itself impossible, causing the cell to cease to function as a cell.
(2) Electrical conductivity. Electrical conductivity is essential because the fuel cell bipolar plate acts as an electrode for the fuel cell.
(3) High dimensional accuracy, that is, thickness accuracy. Electric current flows through the contact between the bipolar plate and the anode or cathode, and therefore poor dimensional accuracy decreases the surface area of contact and degrades electrical conductivity. In addition, poor dimensional accuracy may cause gaps to develop between the fuel cell bipolar plate and the anode or cathode, which can result in cracks in the fuel cell bipolar plate if force is applied to the bipolar plate in such a direction as to compress the gaps. Dimensional accuracy is assessed by measuring the thickness of the fuel cell bipolar plate at predetermined measurement points on a single bipolar plate and obtaining a difference d between a maximum thickness Tmax and a minimum thickness Tmin. The smaller the difference d, the better the performance of the fuel cell.
In order to satisfy the above-described requirements, initially the fuel cell bipolar plate was made by machining of a graphite plate. However, machining took time and the resulting bipolar plate was prohibitively expensive.
More recently, a method of manufacturing the fuel cell bipolar plate has been adopted that uses a powdered raw material composed of a mixture of carbon powder and thermosetting synthetic resin powder. This raw material is poured into a bottom half of a mold (hereinafter “bottom mold”) of a press machine and covered with a top half of the mold (hereinafter “top mold”) thereof, with the press then supplying pressure and heat to form the fuel cell bipolar plate.
However, it is not possible to achieve precise thicknesses for the finished fuel cell bipolar plate with such a method of formation. For example, with the conventional method of formation described above, a difference d described above of 0.2 mm or more has been observed in the dimensional accuracy of a fuel cell bipolar plate of approximately 200 mm a side, which adversely affects the performance of the fuel cell.
In order to solve this problem, Japanese Laid-Open Patent Publication (Kokai) No. 2001-62858 proposes a powdered raw material dropping device having a dropping part having a plurality of downward-facing dropping ports arranged in a matrix-like formation, a slide plate slidably movable between a position at which all the dropping ports are closed and a position at which all the dropping ports are open, and a base that supports such dropping part and slide plate.
Such a method produces much better dimensional accuracy than hitherto existing methods and can produce a uniform thickness. However, because the raw material drops straight down from the dropping ports, the raw material that is deposited tends to concentrate at the center rather than at the periphery, leading to marked differences in the density of the fuel cell bipolar plate if the raw material is viscous.
In addition, Japanese Laid-Open Patent Publication (Kokai) No. 2001-85030 proposes a method in which the powdered raw material is preformed into a tablet and the tablet is then dropped into the mold. According to this method, the powdered raw material can be given a uniform consistency by forming it into a tablet. However, this method necessitates applying a heavy load to the tablet and spreading the material to the edges of the mold when forming the tablet. Consequently, if the powdered raw material is viscous, the edges of the table tend to be neglected when casting an article having a large surface area, thus making it difficult to achieve uniformity. Moreover, non-uniformity tends to occur in the dropping of the tablet into the mold as well, which tends to create uneven thickness.
In an effort to ensure uniformity of thickness and quality, Japanese Laid-Open Patent Publication (Kokai) No. 2004-338268 proposes a method that involves dropping powdered raw material inside a hopper into the bottom mold while moving the hopper from one end of the mold to the other so as to supply powdered raw material up to a certain height.
However, such a method has several problems. Once the dropping of the powdered raw material into the bottom mold stops, the top mold descends and pressure is applied to the powdered raw material, and at the same time the mold is heated and the resin contained in the powdered raw material melts or hardens. Therefore, from at least the second time onward the powdered raw material is dropped into a bottom mold that is already rather hot, and as a result the powdered raw material that drops from the hopper first immediately begins to melt or harden, bringing further movement or hardening to a complete halt and making it difficult to obtain dimensionally consistent castings.
In addition, Japanese Laid-Open Patent Publication (Kokai) No. 2000-77081 proposes a method that involves compressing the powdered raw material into a preliminary casting at low temperature, dropping the preliminary casting into the mold and then compressing and heating it to form the fuel cell bipolar plate.
However, such a method requires a press machine in order to create the preliminary casting. A press is an expensive piece of machinery and its addition turns production facilities into large-scale works, which in turn increases the cost of the fuel cell bipolar plate. Moreover, time is required for the press process that turns the powdered raw material into, a preliminary casting at low temperature. In addition, output is adversely affected by deformation due to spring-back since pressure is applied during preliminary casting, as well as by impurities contaminating the preliminary casting and from damage to the preliminary casting because it is handled directly by hand. Finally, in order to give the finished casting consistently uniform dimensions and density, the preliminary casting itself must be consistently uniform, which is difficult to do.
It should be noted that methods 1-3 described above drop the powdered raw material or the tablet directly into the mold, which takes time and effort and increases the number of steps in the manufacturing process, thereby reducing production efficiency.